replace every letter except the last one using regex

October 31, 2022

I’m relatively new to regex, so bear with me if the question is trivial. I’d like to place a colon between every letter of a string using regex, e.g.,

x <- "ABCD"

I want to get

"A,B,C,D"

It would be nice if I could do that using gsub, sub or related on a vector of strings of arbitrary number of characters.

I tried

> sub("(\\w)", "\\1,", x)
[1] "A,BCD"
> gsub("(\\w)", "\\1,", x)
[1] "A,B,C,D,"
> gsub("(\\w)(\\w{1})$", "\\1,\\2", x)
[1] "ABC,D"

>Solution :

You can use

> gsub("(.)(?=.)", "\\1,", x, perl=TRUE)
[1] "A,B,C,D"

The (.)(?=.) regex matches any char capturing it into Group 1 (with (.)) that must be followed with any single char ((?=.)) is a positive lookahead that requires a char immediately to the right of the current location).

Vriations of the solution:

> gsub("(.)(?!$)", "\\1,", x, perl=TRUE)
## Or with stringr:
## stringr::str_replace_all(x, "(.)(?!$)", "\\1,")
[1] "A,B,C,D"

Here, (?!$) fails the match if there is an end of string position.

See the R demo online:

x <- "ABCD"
gsub("(.)(?=.)", "\\1,", x, perl=TRUE)
# => [1] "A,B,C,D"
gsub("(.)(?!$)", "\\1,", x, perl=TRUE)
# => [1] "A,B,C,D"
stringr::str_replace_all(x, "(.)(?!$)", "\\1,")
# => [1] "A,B,C,D"